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Which statement explicitly names a constraint?（）
A:ALTER TABLE student_grades   ADD   FOREIGN KEY (student_id) REFERENCES students(student_id);
B:ALTER TABLE student_grades   ADD CONSTRAINT NAME = student_id_fk   FOREIGN KEY (student_id) REFERENCES students(student_id);
C:ALTER TABLE student_grades   ADD CONSTRAINT student_id_fk   FOREIGN KEY (student_id) REFERENCES students(student_id);
D:ALTER TABLE student grades   ADD NAMED CONSTRAINT student_id_fk   FOREIGN KEY (student_id) REFERENCES students(student_id);
E:ALTER TABLE student grades   ADD NAME student_id_fk   FOREIGN KEY (student_id) REFERENCES students(student_id);

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出自：Oracle认证

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